1. Field of the Invention
The present invention relates to an optical focusing device, and more particularly to a wavelength selective optical focusing device using the cladding-etching of an optical fiber and an optical module using the same.
2. Description of the Related Art
In order to detect an optical signal traveling through a core of an optical fiber, a path of a core mode of the optical signal transmitted through the core must be changed. However, since the core of the optical fiber is generally coated with a cladding layer having a relative large thickness, the core mode is rarely affected by external factors. For this reason, in order to detect the optical signal from the optical fiber, the optical fiber must be physically deformed. The physical deformation of the optical fiber so that the optical signal is detected from the optical fiber is performed by various methods. One method, which is most easily performed using characteristics of the optical fiber, is the generation of a fiber Bragg grating (FBG) in the core of the optical fiber.
For example, U. S. Pat. No. 5,042,897 entitled “Optical waveguide embedded light redirecting Bragg grating arrangement” discloses an optical fiber, which detects an in-line optical signal using a fiber Bragg grating, as shown in FIG. 1. With reference to FIG. 1, a slanted fiber Bragg grating 11 (having the grating slits spaced at a distance a) is formed in a core 12 so as to radiate a waveguide signal (Rλ) to the outside of the core 12 at an angle α. The above structure is advantageous in that an optical signal traveling through the core 12 is simply radiated to the outside of the optical fiber 10 without an additional device. However, the above structure has a low efficiency in radiating an optical signal to the outside of the core 12, and can be used to monitor an optical signal but is not adequate to detect an optical signal having a high coupling factor. Further, in order to refocus light radiated to the outside of the optical fiber 10, an additional device, such as a lens, is required.
Further, Korean Patent Laid-open Publication No. 2005-8280 entitled “Optical waveguide grating lens and method for manufacturing the same” discloses a focusing waveguide grating coupler (FWGC) for improving a coupling efficiency, as shown in FIG. 2. With reference to FIG. 2, a FWGC 20 comprises a planar waveguide having a substrate cladding layer 21 and a core 22, an upper cladding layer 23 formed on the planar waveguide, a slanted fiber Bragg grating (FBG) 28 formed in the core 22, and a Fresnel grating lens 24 formed on the upper cladding layer 23. In the above structure, a core mode 27 travels along the planar waveguide, and is diffracted by the slanted FBG 28, thereby generating a slanted output beam 25. Then, the slanted output beam 25 is diffracted by the Fresnel grating lens 24, thereby being concentrated onto a focus 26. The above planar waveguide is disadvantageous in that its manufacturing process is complicated and the mode matching with the optical fiber must be correctly achieved so as to minimize loss in transmitting the optical signal. Further, as shown in FIG. 2, slanted FBG 28 of the FWGC 20 is formed in the core 22 by changing a refractive index by causing ultraviolet rays to be incident onto a material sensitive to ultraviolet rays (UV) through a phase mask. However, the above FBG 28 formed using ultraviolet rays (UV) is disadvantageous in that its reproducibility in manufacturing is reduced, aging effects, such as changes in refractive index, are severely generated, and it is difficult to have the maximum optical coupling efficiency above 50%.